The Schengen 90/180 rule is simple to state and genuinely hard to track: 90 days inside the zone within any rolling 180-day window. Simple. Except nobody remembers their exact entry dates from three trips ago. That is why a Schengen 90/180 calculator app exists as a category at all.
Honest comparison, since app names in this niche come and go too fast to rank. The free options all do the same core job: you enter your past and planned entry and exit dates, and the app shows how many of your 90 days you have used and how many remain. That is the whole product. Everything else is packaging.
So judge any candidate on five things. Rolling-window math, done right: it must calculate against any 180-day window, not the calendar year, and if it asks for a year and stops there, it is wrong. An accurate country list: Schengen membership changes, so the app should know which countries count toward your 90 days and which do not, and ideally let you verify the list yourself. Editable history: you will mistype a date, and you need to fix it without starting over. Offline use: you will check this at a border, not on home Wi-Fi. And no account wall for the basic math. A day counter does not need your email.
The free tier of these apps is usually the whole product, and that is fine. This is arithmetic, not advice. But here is what no calculator does: tell you the current entry rules for your passport. A mobile app for counting Schengen days keeps your count honest. It does not keep your information current.
Do the Schengen 90/180 day calculator search, pick a free option that passes the test above, and feed it real dates from your passport stamps. Then, before you book the next trip, get the passport-plus-destination eligibility report at visadocs.fyi, so the rules behind your count are current too.